The toxicity of cleaning substances in liquids, such as process or waste waters, particularly the toxicity of highly volatile, halogenated hydrocarbons and the increasing damage which they cause to the environment, over the last few years has led to ever lower permitted maximum concentrations for these substances. Therefore for the identification and detection of the substances ever improved detection processes and apparatuses are necessary. At present, the substances are detected by gas chromatography and for this purpose highly developed equipment is required, which is complicated with respect to the mechanical, sensor and electronic means used. For chlorinated hydrocarbons use is also made of halogen-specific flame ionization and electron capture detectors. Furthermore special intake systems are used in combination with the detection processes and it is often necessary to concentrate samples beforehand. As a result of the known methods only discontinuous measurements can be performed on samples. However, in the chemical industry halogenated hydrocarbons are vital key chemicals and must therefore also be continuously measurable in numerous process sequences.